The present invention is directed to a detector for light detection for use in devices such as a scanner, and in particular a detector for a scanner using a high energy light source such as a laser, which is capable of scanning a relatively wide web moving at a high speed.
A laser scanner for scanning a wide moving web, that is, a web on the order of greater than 0 to 60" moving at speeds of up to at least 1200 feet per minute is disclosed in the applicant's copending application Serial No. 487,572 filed Mar. 2, 1990.
In a scanning system of that type, the scanner generates a scan line, which is greater than 50" long, across the web and a receiver or detector collects the energy which is either reflected by the web or which passes through the web, generates a signal corresponding thereto, amplifies and conditions the signal and then sends the signal to a control computer where it is analyzed by comparing it with preset thresholds. If the signal from a given point on the scan line exceeds the threshold, an object is reported. One such object is a flaw in the case of a scanner used in a flaw detection system. If the signal is lower than the preset threshold, there is no reported object.
The resolution of the system depends not only on the scanning spot size, but on the response speed of the receiver and the receiver speed is determined by the speed of the photodetector therein and the associated processing circuitry.
In a detector for such a system, there are limitations. High speed electronics yield higher levels of random noise which can cause false triggering in the threshold circuitry. Moreover, there is a tradeoff between speed and cost since certain types of photodetectors are costly, and this cost is multiplied when numerous photodetectors or exotic photodetectors such as photomultipliers are needed to detect light energy from a relatively wide web.